All hail the Rustock botnet takedown. Between March 15 and 17, during which time Rustock was taken down, global spam volumes fell by 33.6%, according to a Symantec MessageLabs Intelligence report. Compared to the week before the takedown, the number of daily spam emails decreased from 52 billion to 33 billion.

At its height, the Rustock botnet pumped out 13.82 billion emails per day, comprising 29% of the world's daily spam diet. But will the Rustock respite last?

"It remains to be seen whether the criminals behind Rustock will be able to recover from this coordinated effort against what has become one of the most technically sophisticated botnets in recent years," said MessageLabs Intelligence senior analyst Paul Wood, in a statement. "Rustock has been a significant part of the botnet and malware landscape since January 2006 -- much longer than many of its contemporaries."

But other botnets are already stepping up to fill the void left by Rustock. Bagle, notably, didn't rank in the top 10 list of the world's worst spam-spewing botnets in 2010, but has become the new leader. It's now generating 8.31 million spam emails daily.

The overall amount of spam originating from botnets also continues to increase. Notably, in March, 83.1% of the world's spam came via botnets, compared with a 2010 average of 88.2%. "Botnets have been and remain a destructive resource for cyber criminals and through the years have become the spammers' air-supply, without which it would be very difficult for them to operate," said Wood.

So far, however, the Rustock takedown -- led by Microsoft, working in conjunction with federal authorities and security firm FireEye -- is serving as a textbook example for how to nuke a botnet. Microsoft's novel takedown strategy, as reported by the Wall Street Journal, alleged that the masterminds behind Rustock were "violating Microsoft trademarks with spam that fraudulently claims Microsoft sponsorships of lotteries and other come-ons." Backed by a court order, authorities physically impounded the command-and-control (C&C) servers that powered the botnet.

Authorities are now seeking Rustock's masterminds. "It does not look like there were more than a couple of people running it to me," Alex Lanstein, a senior engineer at security firm FireEye, told the BBC. They were also crafty. "By locating all the C&C servers in middle-America, not in major metropolitan areas, they were able to stay off the radar," he said.

But placing C&C servers in a country that respects takedown notices is unusual. Indeed, most botnets appear to be based either in Russia or the Ukraine, and especially on so-called bulletproof hosting services, which ignore takedown notices and would only cost about 50% more per month in subscription fees, compared with regular high-volume hosting services.

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